Integrated EMI filter circuit with ESD protection and incorporating capacitors

ABSTRACT

An integrated electromagnetic interference (EMI) filter circuit with electrostatic discharge (ESD) protection and incorporating capacitors is provided. At least one passive element, i.e. resistor or inductor is connected between an input terminal and an output terminal. A first capacitor is connected between ground and the input terminal, and a second capacitor is connected between ground and the output terminal. A first diode and a second diode are connected in parallel to the first capacitor and the second capacitor. One or multiple parallel capacitors are connected in parallel to the passive element and between the input terminal and the output terminal for frequency compensation by employing the novel EMI LPF circuit, it is extraordinarily advantageous of enhancing its rejection band attenuation and meanwhile maintaining high cut-off frequency while implementation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an Electromagnetic Interference (EMI)low-pass filter (LPF) integrated circuit (IC), and more specifically toa LPF IC with integrated electrostatic discharge (ESD) protection andincorporating a capacitor network to enhance filter performance.

2. Description of the Prior Art

EMI interferences associated with cellular phones, radios, televisionsand other electronic systems widely exist and can seriously affectnormal operation of electronic products, especially in high resolutionand high data rate electronics such as smart phones, laptops, LCDdisplays, high-definition TV, digital cameras, etc. To minimize theseinterferences, EMI LPF filters are widely used in many electronicproducts to block incoming radio frequency (RF) interferers and noises.On the other hand, electronic systems can generate and emit unwanted RFnoises that will affect other electronics. It is hence critical to blockany incoming and emitting EMI noises of electronics.

Using EMI filters in data lines and I/O ports is an effective way toblock the EMI radiation in both directions by filtering the unwanted RFnoises generated from electronic systems and/or occurring in the naturalenvironments. In particular, wireless electronics such as smart phonesand LCD are the victims of EMI effect. The data lines linking the maincircuit board to the display panel are usually long and very susceptibleto high frequency EMI interferers. Meanwhile, any RF noises generatedfrom wireless devices can radiate into the environment through the datalines, which act like antennas. Therefore, the two major tasks for EMIfilter are to block any high frequency RF noises and to maintain thebase band signal integrity.

LPF is utilized to resolve the above EMI problems. The criticalspecifications for LPF EMI filter design include low pass-band insertionloss (IL), a broad pass band and high rejection-band attenuation. A lowinsertion loss in the low pass band ensures the desired baseband signalspassing through the LPF filter with minimum signal loss. A broad passband, defined as the frequency bandwidth from DC (e.g., 1 MHz) to thecut-off frequency (f_(c)) measured at the −3 dB insertion loss point,allows the desired baseband signals with wider frequency spectrum (i.e.,lots of useful baseband harmonic signals) to pass through the filter.Generally, a wider pass-band, i.e., a higher f_(c), enables higherwireless communication data rates. The baseband operation frequency isusually ⅕ to ⅓ of the f_(c), which guarantees good signal integrity. Therejection-band is determined by the wireless systems, typically from 800MHz to 6 GHz. The rejection band serves to remove any high-frequency EMIinterferers, which are generally associated with the carrier bandfrequencies in RF systems. To ensure the desired data rates and signalintegrity, at least −20 dB attenuation in the rejection-band for the EMIinterferes is required in EMI filter circuit designs to ensure therequired signal-to-noise ratio (SNR) for the wireless systems.

FIG. 1 illustrates a conventional S-parameter measurement result for theforward transmission gain parameter (S₂₁) of an EMI LPF showing its keyspecifications (i.e., specs), e.g., IL, f_(c), and rejection bandbehaviors. The insertion loss curve is always affected by parasiticparameters induced by the package and printed circuit board (PCB),causing the S₂₁ curve bounce back in the rejection band.

Traditionally, integrated resistor-capacitor (RC) and inductor-capacitor(LC) filters are used as EMI filters. Such EMI filters usuallyincorporate integrated ESD protection devices to ensure system leveltransient voltage suppression (TVS) function. FIG. 2A and FIG. 2B showthe two ideal π-type CRC and CLC EMI LPF filter circuits, respectively.FIG. 3A and FIG. 3B illustrate these two CRC and CLC EMI filter circuitswith integrated ESD protection, which are Zener diodes in the exampleswhere the anodes are connected to the input and output nodes,respectively, and the cathodes are grounded. The parasitic capacitanceof the ESD diodes contribute to the two capacitors required, whichrequires careful design balance between the required capacitance valuesand the ESD diode sizes for given ESD protection level. The Zener diodeESD protection devices can be replaced by any other type of ESDprotection structures in practical designs.

As a result, the basic EMI filters cannot deliver strong rejection bandattenuation and very high f_(c) to ensure excellent baseband signalintegrity due to LPF filter design trade-offs. This is because, inpractical LPF filter designs, to achieve the required low insertion lossand broad pass-band, while obtaining high rejection-band attenuation,are in conflict and challenging. Particularly, the S₂₁ curve should havea very clean and high f_(c) and a fast roll-off attenuation curve, i.e.,a steep S₂₁ roll-off curvature after the designed f_(c) point. Theconventional single-stage CRC and CLC filters shown in FIGS. 2A, 2B, 3Aand 3B cannot achieve these requirements due to various IC and packageparasitic effects. Alternatively, multiple-stage CRC and CLC EMI filtercircuits may improve the rejection band attenuation while achieving highf_(c). FIG. 4A and FIG. 4B depict examples of two-stage CRC and CLC LPFcircuits for this purpose, respectively. The extra frequency polesintroduced by the extra capacitors and inductors in the LPF circuits canbe fine-tuned to compensate frequency behaviors, hence to achieve therequired f_(c), sharper roll-off curve and better rejection bandattenuation. Similarly, higher-order multiple-stage LPF filter circuitscan further improve the EMI filter performance through frequencycompensation. Unfortunately, a multiple-stage LPF filter requires extracomponents, including resistors, capacitors or inductors, resulting infast increase of the EMI LPF filter size, which is undesired for mostsmall footprint electronics, such as smart phones.

On account of above, it should be obvious that there is indeed an urgentneed for a new EMI circuit which can enhance the rejection bandattenuation, while maintaining high f_(c) and yet without increase ofthe filter size when using an EMI LPF filter.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned disadvantages, an approach inaccordance with the present invention is provided for a new EMI LPFfilter IC incorporating at least one parallel capacitor, which enhancesthe rejection band attenuation, while maintaining high f_(c). Accordingto the present invention, the new EMI circuit may be applied to π-typecapacitor-resistor-capacitor (CRC) filters and/or π-typecapacitor-inductor-capacitor (CLC) filters of various formats.

In one aspect, the EMI LPF circuit incorporating ESD protection of thepresent invention comprises: at least one passive element, a firstcapacitor and a second capacitor, a first diode and a second diode, andat least one parallel capacitor.

The at least one passive element is connected between an input terminaland an output terminal. The first capacitor is connected between groundand a node of the at least one passive element and the input terminal,and the second capacitor is connected between ground and a node of theat least one passive element and the output terminal. An anode and acathode of the first diode are respectively connected to the inputterminal and ground, and an anode and a cathode of the second diode arerespectively connected to the output terminal and ground. The firstcapacitor, the second capacitor, the first diode and the second diodeare connected in parallel.

The at least one parallel capacitor is connected in parallel to the atleast one passive element and between the input terminal and the outputterminal for frequency compensation.

According to one embodiment of the present invention, the at least onepassive element can be an inductor or a resistor.

According to one embodiment of the present invention, the at least oneparallel capacitor can be a metal-oxide-semiconductor (MOS) capacitor, apolysilicon-insulator-polysilicon (PIP) capacitor, or ametal-insulator-metal (MIM) capacitor.

A physical layout for the at least one parallel capacitor can be made invarious ways, for example, a lateral metal-metal finger capacitor and/ora vertical metal-metal overlap capacitor.

According to one embodiment of the present invention, when the LPF EMIfilter is applied to a two-stage CRC circuit, it further comprises athird capacitor and a third diode, wherein two resistors are connectedin series between the input terminal and the output terminal, the thirdcapacitor is connected between ground and a node of the first resistorand the second resistor, and an anode and a cathode of the third diodeare respectively connected to the node of the first resistor and thesecond resistor and ground.

According to one embodiment of the present invention, when the LPF EMIfilter is applied to a three-stage CRC/CLC circuit, it further comprisesa third capacitor and a fourth capacitor, wherein threeresistors/inductors are connected in series between the input terminaland the output terminal, the third capacitor is connected between groundand a node of the first resistor/inductor and the secondresistor/inductor, and the fourth capacitor is connected between groundand a node of the second resistor/inductor and the thirdresistor/inductor.

These and other objectives of the present invention will become obviousto those of ordinary skill in the art after reading the followingdetailed description of preferred embodiments.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings:

FIG. 1 shows a typical S₂₁ curve for a traditional EMI LPF filter.

FIG. 2A shows a schematic view of conventional π-type CRC EMI LPF filtercircuit.

FIG. 2B shows a schematic view of conventional π-type CLC EMI LPF filtercircuit.

FIG. 3A shows a schematic view of the conventional π-type CRC EMI LPFfilter circuit with integrated ESD protection Zener diode.

FIG. 3B shows a schematic view of the conventional π-type CLC EMI LPFfilter circuit with integrated ESD protection Zener diode.

FIG. 4A shows a schematic view of the conventional two-stage CRC EMI LPFfilter circuit with integrated ESD protection Zener diode.

FIG. 4B shows a schematic view of the conventional two-stage CLC EMI LPFfilter circuit with integrated ESD protection Zener diode.

FIG. 5 shows a schematic view of an EMI LPF filter circuit incorporatingESD protection implemented in a single-stage CLC schematic in accordancewith one embodiment of the present invention.

FIG. 6 is a drawing which compares the S₂₁ characteristics of thesingle-stage CLC EMI filter shown in FIG. 5 with its counterpart withoutusing the parallel capacitor in FIG. 3B.

FIG. 7A shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a two-stage CRC schematic inaccordance with one embodiment of the present invention.

FIG. 7B shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a two-stage CRC schematic inaccordance with another embodiment of the present invention.

FIG. 8 is a drawing which compares the S₂₁ characteristics of the EMILPF filter in FIG. 7A with its counterpart without using the parallelcapacitor in FIG. 4A.

FIG. 9A shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a three-stage CRC schematicin accordance with one embodiment of the present invention.

FIG. 9B shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a three-stage CLC schematicin accordance with one embodiment of the present invention.

FIG. 9C shows a schematic view of a conventional three-stage CRC EMI LPFfilter circuit without using a parallel capacitor.

FIG. 10 is a drawing which compares the S₂₁ characteristics of the EMILPF filter in FIG. 9A with its counterpart without using the parallelcapacitor in FIG. 9C.

FIG. 11A shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a three-stage CRC schematicemploying two parallel capacitors in accordance with one embodiment ofthe present invention.

FIG. 11B shows a schematic view of an EMI LPF filter circuitincorporating ESD protection implemented in a three-stage CLC schematicemploying three parallel capacitors in accordance with one embodiment ofthe present invention.

FIG. 11C shows a schematic view of a conventional three-stage CLC EMILPF filter circuit without using a parallel capacitor.

FIG. 12 is a drawing which compares the S₂₁ characteristics of the EMILPF filter in FIG. 11B with its counterpart without using the parallelcapacitor in FIG. 11C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

The embodiments described below are illustrated to demonstrate thetechnical contents and characteristics of the present invention and toenable the persons skilled in the art to understand, make, and use thepresent invention. However, it shall be noticed that, it is not intendedto limit the scope of the present invention. Therefore, any equivalentmodification or variation according to the spirit of the presentinvention is to be also included within the scope of the presentinvention.

In order to achieve the purpose of good rejection band attenuation andsteep roll-off at high f_(c), the proposed structure of the presentinvention provides a novel EMI LPF IC filter, which incorporates atleast one parallel capacitor such that the EMI LPF IC incorporatingcapacitors can be applied to single-stage and/or multiple-stage CRC andCLC circuits in different schematics, and meanwhile maintainingexcellent ESD protection.

Refer to FIG. 5, which shows an EMI LPF filter circuit incorporating ESDprotection implemented in a single-stage CLC schematic in accordancewith one embodiment of the present invention.

The EMI LPF filter circuit 1 of the first embodiment includes two ESDprotection diodes D1 and D2, two capacitors C1 and C2, a parallelcapacitor Cp, and a passive element connected between an input terminalVin and an output terminal Vout. In the first embodiment of the presentinvention, the passive element connected between the input terminal Vinand the output terminal Vout is a channel inductor L, hence forming aCLC type EMI filter. In other embodiments of the present invention, thepassive element can also be implemented by using a resistor or moreresistors to form a CRC type EMI filter. The explanatory schematic (FIG.5) is not intended to limit the scope of the present invention. Anyequivalent modification or variation, i.e. replacing the inductor with aresistor according to the spirit of the present invention is to be alsoincluded within the scope of the present invention.

According to the first embodiment of the present invention, D1 can be aZener diode with its anode connected to the input terminal Vin andcathode connected to the ground. D2 is a Zener diode with its anodeconnected to the output terminal Vout and the cathode connected to theground. The parasitic capacitance of D1 and D2 contribute to the tworequired capacitors C1 and C2 of the EMI filter. The first capacitor C1is connected between the ground and a node of the inductor L and theinput terminal Vin. The second capacitor C2 is connected between theground and a node of the inductor L and the output terminal Vout. Thetwo ESD protection diodes D1, D2 and the two capacitors C1, C2 areconnected in parallel.

The parallel capacitor Cp is connected in parallel to the inductor L andalso between the input terminal Vin and the output terminal Vout. In oneembodiment, the parallel capacitor Cp can be formed as a MOS capacitor,a PIP capacitor, or a MIM capacitor. And, a physical layout for theparallel capacitor Cp can be made in various ways including a lateralmetal-metal finger capacitor or a vertical metal-metal overlapcapacitor. The input and output terminals are connected to a packagelead-frame by bonding wires. The ESD diodes must be isolated from eachother and any other components.

FIG. 6 compares the S₂₁ characteristics of the new single-stage CLC EMIfilter shown in FIG. 5 with its counterpart without using the parallelcapacitor Cp in FIG. 3B, wherein the thick line indicates the S₂₁characteristics of FIG. 3B, while the thin line indicates the S₂₁characteristics of FIG. 5.

The input signal coming to the input terminal Vin passes through the LPFfilter to the output terminal Vout with minimum low-frequency insertionloss (IL) within the pass band. The pass band covers the frequencyspectrum from DC to f_(c), corresponding to the −3 dB drop-off point.Good signal integrity typically requires the base band operationfrequency ranges from ⅕ to ⅓ of the f_(c). Consequently, any incomingsignal with the frequency higher than f_(c) will be blocked by the LPFfilter due to rejection band attenuation. The rejection band attenuationshould be at least −20 dB to achieve the desired high SNR ratio.

As shown in FIG. 6, it is clear that, due to frequency compensation bythe Cp, the proposed EMI LPF circuit 1 of the present invention achievesmuch improved high-frequency rejection band attenuation withoutaffecting its low-frequency insertion loss, i.e., the attenuation overthe rejection band from 800 MHz to 6 GHz is significantly steeper whilethe f_(c) stays unchanged. By carefully tuning the inductor L1 and theparallel capacitor Cp, a suitable resonant frequency point can beselected to tune the lowest point of rejection band.

Refer to FIG. 7A for the second embodiment of the present invention,which shows an EMI LPF filter circuit incorporating ESD protectionimplemented in a two-stage CRC schematic in accordance with oneembodiment of the present invention.

The EMI LPF filter circuit 2 of the second embodiment includes three ESDprotection diodes D1, D2 and D3, three capacitors C1, C2 and C3, aparallel capacitor Cp, and two passive elements connected between aninput terminal Vin and an output terminal Vout. In the second embodimentof the present invention, the two passive elements connected between theinput terminal Vin and the output terminal Vout are a first resistor R1and a second resistor R2 which connects in series with the firstresistor R1, forming CRC type EMI filters. Similar to the firstembodiment, the passive elements defined in the second embodiment canalso be implemented by using an inductor or more inductors to form CLCtype EMI filters. The explanatory schematic (FIG. 7A) is not intended tolimit the scope of the present invention. Any equivalent modification orvariation, i.e. replacing the resistors with inductors according to thespirit of the present invention is to be also included within the scopeof the present.

According to the second embodiment of the present invention, D1 is aZener diode with its anode connected to the input terminal Vin andcathode connected to the ground. D2 is a Zener diode with its anodeconnected to the output terminal Vout and the cathode connected to theground. D3 is a Zener diode with its anode connected to a node of thefirst resistor R1 and the second resistor R2, and the cathode connectedto the ground. The parasitic capacitance of D1, D2 and D3 contribute tothe three required capacitors C1, C2 and C3 of the EMI filter, whereinC1 is connected between the ground and a node of the first resistor R1and the input terminal Vin; C2 is connected between the ground and anode of the second resistor R2 and the output terminal Vout; and C3 isconnected between the ground and a node of the first resistor R1 and thesecond resistor R2. The ESD protection diodes D1, D2, D3 and the threecapacitors C1, C2, C3 are connected in parallel.

The parallel capacitor Cp is connected in parallel to the first andsecond resistors R1, R2 and between the input terminal Vin and theoutput terminal Vout. In one embodiment, the parallel capacitor Cp canbe formed as a MOS capacitor, a PIP capacitor, or a MIM capacitor, and aphysical layout for the parallel capacitor Cp can be made in variousways including a lateral metal-metal finger capacitor or a verticalmetal-metal overlap capacitor.

In this embodiment, when ESD transients occur to the input terminal Vin,the high ESD current will be shunted by the three ESD protection diodesD1, D2 and D3 to protect the internal circuit. Because of the resistorsR1 and R2 in the channel between input and output terminals, themajority of the ESD surge will discharge into the ground through D1. Theoutput voltage will be clamped to a safe level by the ESD protectiondiodes. Compared to a traditional single-stage EMI LPF filter, thistwo-stage EMI LPF filter provides better ESD protection to internalcircuits while maintaining the same channel resistance and capacitance.For a single-stage CRC filter, though the majority ESD currentdischarges through the first ESD protection diode, a sizable residuecurrent will pass through the second ESD diode and determines thecritical output clamping voltage. However, according to the newtwo-stage CRC LPF filter of the present invention, the first two ESDprotection diodes (D1, D3) serve to discharge the large ESD currentsurge and the residue ESD current passing through the third ESDprotection (D2) can be significantly reduced, therefore, the outputclamping voltage will be much lower, which is one major advantage of thenew two or multi-stage EMI filter circuits.

FIG. 8 compares the S₂₁ characteristics of the new EMI LPF filter inFIG. 7A with its counterpart without using the parallel Cp in FIG. 4A,wherein the thick line indicates the S₂₁ characteristics of FIG. 4A,while the thin line indicates the S₂₁ characteristics of FIG. 7A. Asshown in FIG. 8, it is clear that, due to frequency compensation by theparallel capacitor Cp, the new EMI filter achieves much improvedhigh-frequency rejection band attenuation without affecting itslow-frequency insertion loss, i.e., the attenuation over the rejectionband from 800 MHz to 6 GHz is significantly steeper while the f_(c)stays unchanged.

Moreover, refer to FIG. 7B, which shows a third embodiment of thepresent invention, illustrating an EMI LPF filter circuit incorporatingESD protection implemented in a two-stage CRC schematic in accordancewith another embodiment of the present invention.

In the embodiments of FIG. 7B, the EMI LPF filter circuit implemented intwo-stage CRC schematic 3 uses more than one parallel capacitor disposedbetween the input and output terminal.

As shown in FIG. 7B, the parallel capacitor is implemented by using afirst parallel capacitor Cp1 and a second parallel capacitor Cp2,wherein the first parallel capacitor Cp1 is connected to the inputterminal Vin and a node of R1 and R2, and the second parallel capacitorCp2 is connected to the output terminal Vout and the node of R1 and R2.In other words, according to the third embodiment of the presentinvention, the two parallel capacitors Cp1 and Cp2 can also be used inparallel with each resistor R1 and R2 between the input and outputterminal so as to achieve the same objective of the present invention,i.e. an attenuation in rejection band and steep roll-off at high f_(c).

Refer to FIG. 9A and FIG. 9B for the fourth and fifth embodiment of thepresent invention, which respectively shows an EMI LPF filter circuitincorporating ESD protection implemented in a three-stage CRC and CLCschematic in accordance with one embodiment of the present invention.

As shown in the fourth and fifth embodiment of the present invention,the EMI LPF filter circuit 4 and 5 comprises two ESD protection diodesD1 and D2, four capacitors C1, C2, C3 and C4, a parallel capacitor Cp,and three passive elements connected between the input terminal Vin andthe output terminal Vout. In the fourth embodiment shown in FIG. 9A, thethree passive elements are serially connected resistors R1, R2 and R3.In the fifth embodiment shown in FIG. 9B, the three passive elements areserially connected inductors L1, L2 and L3. According to the presentinvention, other simple combinations of serially-connected resistors orinductors can also be implemented so that the at least one passiveelement is composed. The proposed invention is fully describedabovementioned but not limited thereto. In FIGS. 9A and 9B, similar tothe previous embodiments, the two diodes D1 and D2 are Zener diodes. Thefirst capacitor C1 is connected between the ground and a node of thefirst resistor R1 (the first inductor L1 in FIG. 9B) and the inputterminal Vin; C2 is connected between the ground and a node of the thirdresistor R3 (the third inductor L3 in FIG. 9B) and the output terminalVout; C3 is connected between the ground and a node of the firstresistor R and the second resistor R2 (the first inductor L1 and thesecond inductor L2 in FIG. 9B); and C4 is connected between the groundand a node of the second resistor R2 and the third resistor R3 (thesecond inductor L2 and the third inductor L3 in FIG. 9B). The ESDprotection diodes D1, D2 and the four capacitors C1-C4 are connected inparallel.

The parallel capacitor Cp is connected in parallel to the resistors R1,R2, R3 (or the inductors L1, L2 and L3 in FIG. 9B) and between the inputterminal Vin and the output terminal Vout for frequency compensation.FIG. 10 compares the S₂₁ characteristics of the new EMI LPF filter inFIG. 9A with its counterpart without using the parallel Cp in FIG. 9C,wherein the solid thick line indicates the S₂₁ characteristics of FIG.9C while the dotted thin line indicates the S₂₁ characteristics of FIG.9A. According to the present invention, the Cp based frequencycompensation serves to significantly improve the rejection bandperformance without affecting its low-frequency insertion loss. Hence,as shown in FIG. 10, it is apparent that adding the parallel capacitorCp in the three-stage CRC EMI filter results in much fasterhigh-frequency roll-off speed in the rejection band compared to itscounterpart circuit without the Cp shown in FIG. 9C.

Apart from above, other embodiments of the proposed invention considerthe use of multiple parallel capacitors between the input and outputterminal. These embodiments are shown in FIG. 11A and FIG. 11B, whereinFIG. 11A shows a sixth embodiment of the present invention, illustratingan EMI LPF filter circuit incorporating two parallel capacitorsimplemented in a three-stage CRC schematic, and FIG. 11B shows a seventhembodiment of the present invention, illustrating an EMI LPF filtercircuit incorporating three parallel capacitors implemented in athree-stage CLC schematic.

In FIGS. 11A and 11B, the parallel capacitors can be put parallel toone, two or more resistors and/or inductors when applied in a multiplestage CRC and/or CLC EMI filter. According to the embodiment of FIG.11A, the first parallel capacitor Cp1 is connected to the input terminalVin and a node of R2 and R3. The second parallel capacitor Cp2 isconnected to the output terminal Vout and a node of R1 and R2. By tuningthe parameter of different parallel capacitors, its S₂₁ curve can alsobe improved.

Similarly, a distributed multiple parallel capacitor schematic is usedto realize the required frequency compensation as shown in FIG. 11B. Thefirst parallel capacitor Cp1 is connected to the input terminal Vin anda node of L1 and L2, the third parallel capacitor Cp3 is connected tothe output terminal Vout and a node of L2 and L3, and the secondparallel capacitor Cp2 is connected between the first parallel capacitorCp1 and the third parallel capacitor Cp3. These parallel capacitors canalso be formed as MOS capacitors, PIP capacitors, or MIM capacitors, anda physical layout for them can also be made in various ways includinglateral metal-metal finger capacitors or vertical metal-metal overlapcapacitors as previously mentioned.

FIG. 12 compares the S₂₁ characteristics of the new EMI LPF filter inFIG. 11B with its counterpart without using the parallel Cp in FIG. 11C,wherein the thick line indicates the S₂₁ characteristics of FIG. 11C,while the thin line indicates the S₂₁ characteristics of FIG. 11B.Compared with the conventional three-stage LC filter shown in FIG. 11C,the rejection band attenuation performance of the present invention ismuch improved as shown in FIG. 12, and the cut-off frequency is almostnot changed.

As a result, to sum up, a novel EMI LPF IC filter incorporating ESDprotection has been provided in the present invention to enhance therejection band attenuation and to maintain high cut-off frequency. Byusing at least one parallel capacitor connected to a conventional EMILPF circuit, the Cp based frequency compensation serves to significantlyimprove its rejection band performance without affecting itslow-frequency insertion loss. Moreover, a steep roll-off at high f_(c)is achieved by employing the present invention. The proposed integratedEMI LPF filter circuit incorporating one or multiple parallel capacitorscan be applied to single-stage and/or multiple-stage CRC and CLCcircuits in different schematics, and meanwhile maintaining an excellentESD protection effect. The Zener diode ESD protection devices describedas the example in this invention can be replaced by any other type ofESD protection structures in practical designs, for example, asilicon-controlled rectifier (SCR) structure, etc.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the scope or spirit of the invention. In view ofthe foregoing, it is intended that the present invention covermodifications and variations of this invention provided they fall withinthe scope of the invention and its equivalent.

What is claimed is:
 1. An integrated electromagnetic interference (EMI)filter circuit with electrostatic discharge (ESD) protection andincorporating capacitors, comprising: a) at least one passive elementconnected between an input terminal and an output terminal; b) a firstcapacitor and a second capacitor, wherein said first capacitor isconnected between ground and said input terminal, and said secondcapacitor is connected between ground and said output terminal; c) afirst diode and a second diode, wherein an anode and a cathode of saidfirst diode are respectively connected to said input terminal andground, an anode and a cathode of said second diode are respectivelyconnected to said output terminal and ground, and said first capacitor,said second capacitor, said first diode and said second diode areconnected in parallel; d) at least one parallel capacitor connected inparallel to said at least one passive element and between said inputterminal and said output terminal for frequency compensation; and e) athird capacitor and a fourth capacitor when said at least one passiveelement comprises first, second, and third passive elements connected inseries between said input terminal and said output terminal, whereinsaid third capacitor is connected between ground and a node of saidfirst and second passive elements, and said fourth capacitor isconnected between round and a node of said second and third passiveelements, wherein said at least one parallel capacitor comprises a firstand second parallel capacitors, said first parallel capacitor beingconnected to said input terminal and said node of said second and thirdpassive elements, and said second parallel capacitor is connected tosaid output terminal and said node of said first and second passiveelements.
 2. The integrated EMI filter circuit of claim 1, wherein saidfirst diode and said second diode comprise at least one of Zener diodesand a silicon-controlled rectifier (SCR) structure.
 3. The integratedEMI filter circuit of claim 1, wherein said at least one passive elementcomprises an inductor.
 4. The integrated EMI filter circuit of claim 1,wherein said at least one passive element comprises a resistor.
 5. Theintegrated EMI filter circuit of claim 1, wherein said at least oneparallel capacitor comprises at least one of a metal-oxide-semiconductor(MOS) capacitor, a polysilicon-insulator-polysilicon (PIP) capacitor,and a metal-insulator-metal (MIM) capacitor.
 6. The integrated EMIfilter circuit of claim 1, further comprising a third capacitor and athird diode when said at least one passive element comprises a firstpassive element and a second passive element, wherein said first passiveelement and said second passive element are connected in series betweensaid input terminal and said output terminal, said third capacitor isconnected between ground and a node of said first passive element andsaid second passive element, and an anode and a cathode of said thirddiode are respectively connected to said node of said first passiveelement and said second passive element and ground.
 7. The integratedEMI filter circuit of claim 6, wherein said at least one parallelcapacitor further comprises a first parallel capacitor and a secondparallel capacitor, said first parallel capacitor is connected to saidinput terminal and said node of said first passive element and saidsecond passive element, and said second parallel capacitor is connectedto said output terminal and said node of said first passive element andsaid second passive element.
 8. The integrated EMI filter circuit ofclaim 7, wherein said first parallel capacitor and said second parallelcapacitor comprise at least one of a MOS capacitor, a PIP capacitor, anda MIM capacitor.
 9. The integrated EMI filter circuit of claim 6,wherein said third diode comprises at least one of a Zener diode and asilicon-controlled rectifier (SCR) structure.
 10. The integrated EMIfilter circuit of claim 6, wherein said first passive element and saidsecond passive element comprise inductors.
 11. The integrated EMI filtercircuit of claim 6, wherein said first passive element and said secondpassive element comprise resistors.
 12. The integrated EMI filtercircuit of claim 1, wherein said first passive element, said secondpassive element and said third passive element comprise inductors. 13.The integrated EMI filter circuit of claim 1, wherein said first passiveelement, said second passive element and said third passive elementscomprise resistors.
 14. The integrated EMI filter circuit of claim 1,wherein said first parallel capacitor and said second parallel capacitorcomprise at least one of a MOS capacitor, a PIP capacitor, and a MIMcapacitor.
 15. The integrated EMI filter circuit of claim 1, whereinsaid at least one parallel capacitor further comprises a first parallelcapacitor, a second parallel capacitor and a third parallel capacitor,said first parallel capacitor is connected to said input terminal andsaid node of said first passive element and said second passive element,said third parallel capacitor is connected to said output terminal andsaid node of said second passive element and said third passive element,and said second parallel capacitor is connected between said firstparallel capacitor and said third parallel capacitor.
 16. The integratedEMI filter circuit of claim 15, wherein said first parallel capacitor,said second parallel capacitor and said third parallel capacitorcomprise at least one of a MOS capacitor, a PIP capacitor, and a MIMcapacitor.